Hi Baron, I've experience with both free air and fan on a 503 and would not go down the free air path again. The primary reason relates to on ground movement where a long transit from hangar to threshold and the start of my take off role caused over heating and costly engine damage when relying on free air.
In my case early in my flying hours, the cylinder head bolts snapped and the piston destroyed the cylinder wall at 1500 feet elevation plus 3 miles after take-off. This was enough prompting to make me seek the advice of Eddie the Expert, Gary at Bert Flood Imports. No matter how cleverly I framed my question to trick him into endorsing Free Air, Gary seemed to frame his response the same (and I'm paraphrasing here), "the fan is of part of the 503 for a reason you idiot". Also he seemed to suggest that fuel mix, fresh fuel, good filters, guages, plugs and all else per Rotax operators manual contributed to good safe performance.
Strangely, since operating with the fan and factory cowlings, the overheating issue has resolved.
If the relatively minor amount of power expended to drive the fan effects your aircrafts overall performance a motor upgrade may be required.
Many pilots operate with free air with no problems. Of course there are generally many solutions to engineering problems, however Rotax built in a solution for temperature issues and trying to reinvent the wheel so to speak is a waste of energy which adds to down time and cost; that's my experience.
By the way, it took a couple of rebuilds before the message finally sank in. Cheers, Paul